1. Field of Invention
The present invention relates to an epitaxial composite comprising a thin film of single crystal Group III-V compound semiconductor such as GaAs, GaP or GaAs.sub.1-x P.sub.x, Ga.sub.1-y Al.sub.y As, Ga.sub.1-y In.sub.y As, AlAs . . . ; InAs . . . and other solutions thereof on a single crystal, electrically insulating substrate. The invention also relates to a process for producing in situ the heteroepitaxial composite by reaction of an appropriate organic compound of the Group III constituent of the semiconductor in an atmosphere containing the Group V constituent of the semiconductor and/or its hydride.
2. Description of the Prior Art
As microelectronic technology improves, the search for better semiconductor materials becomes more intense. While past process technology has concentrated on the use of germanium and silicon for the formation of semiconductor devices, other semiconductor materials are becoming increasingly more important. Gallium arsenide (GaAs) and other zinc-blend III-V semiconductor compounds and alloys are considered to be among the most versatile of all semiconductor materials. For example, varactors, transistors, microwave diodes, light-emitting diodes, injection lasers, bulk microwave power sources, negative resistance amplifiers, and bulk-effect integrated circuits are all possible with gallium arsenide.
Although GaAs, Ge and Si all exhibit semiconducting properties, the differences between GaAs and the elemental semiconductors Ge and Si enhance gallium arsenide's potential usefulness. In GaAs, the minimum of the conduction band and the maximum of the valence band are such that direct electronic transitions can occur between the bands, allowing gallium arsenide to be used, for example, as an injection laser. This is not true for silicon or germanium.
Gallium arsenide has a higher electron mobility and a wider band gap than either germanium or silicon. Further, GaAs has two valleys in its conduction band, separated by an energy difference. Electrons in the lower-energy valley have a higher mobility than those in the higher-energy valley. As the voltage across a sample of GaAs is increased, more electrons are excited into the upper, lower-mobility valley and the current decreases, causing bulk instabilities such as the Gunn effect. This permits GaAs to be used for microwave power sources of types not possible for either silicon or germanium.
In the past, the primary obstacle to more extensive exploitation of GaAs has been its relative impurity compared with either germanium or silicon. These elemental semiconductor materials can be zone-refined in a vacuum after growth, but such purification has not been possible with GaAs. This inpurity problem has meant; e.g., that high quality transistors could not be fabricated with previously available GaAs materials.
Two methods of crystal growth, the Czochralski and the horizontal Bridgman, may be used to grow from a melt bulk single crystal GaAs of relatively high purity. However, it is widely accepted that to obtain optimum purity, device-grade gallium arsenide, growth from the vapor phase is a preferred method of growth.
In the past, several techniques have been used to grow GaAs from the vapor phase onto GaAs substrates. Two such methods are described in the article entitled "Film-Making: A Delicate Job Performed Under Pressure" by Kenneth L. Lawley, Electronics, Nov. 13, 1967, beginning at page 114. In one technique described therein, bulk gallium arsenide is placed in one region of a horizontal reaction chamber and heated to a temperature of about 775 C. (775 degrees Centigrade). A chlorine source gas is introduced into the chamber which reacts with vaporized gallium to form GaCl.sub.3. In a central region of the chamber, at a temperature of about 850 C., the GaCl.sub.3 is mixed with arsine (AsH.sub.3). The mixture then is passed over a GaAs substrate situated in a third region of the chamber heated to a temperature of about 750 C. The trichloride and the arsine each dissociate and combine to form an epitaxial GaAs film on the GaAs substrate.
Another technique described by Lawley requires the presynthesis of a mixture of GaAs in liquid gallium. This presynthesis is achieved in a horizontal reaction chamber exposing liquid gallium to arsenic trichloride. If the arsenic vapor pressure exceeds the decomposition pressure of the GaAs-in-gallium solution at the operating temperature of about 850 C., a gallium arsenide skin forms on the gallium. This skin serves as the source of gallium arsenide for deposition onto a GaAs substrate located in another portion of the chamber and heated to a temperature (between 700 C. and 750 C.) lower than the temperature (800 C. to 850 C.) of the GaAs-in-gallium source.
These prior art vapor phase GaAs growth techniques suffer various shortcomings. First, the techniques all require the use of reaction chambers in which two or more regions of the chamber are heated to different, closely controlled temperatures. Such a multiple temperature requirement is difficult to implement in a production facility. Further, the prior art techniques each require use of gallium metal as a source material present in the deposition chamber. Gallium metal is difficult to obtain free of impurities because of its reactivity at high temperature with its container, and these impurities tend to vaporize in the chamber and contaminate the deposited GaAs film.
Moreover, all previous reports of successful vapor phase single crystal GaAs deposition have been on non-electrically-insulated substrates. A heteroepitaxial composite comprising a continuous film of single crystal III-V compound such as GaAs, GaP or the alloys; e.g., GaAs.sub.1-x P.sub.x, Ga.sub.1-y Al.sub.y As, on a monocrystalline, electrically insulating substrate has not been achieved in the prior art. Such a composite is extremely valuable. For example, by producing single crystal gallium arsenide on an electrically insulating substrate, by performing appropriate device fabrication steps well known in the state-of-the-art, such as diffusion, epitaxial growth while dopant is added to the growing layer, photolithography, and other device fabrication steps, and subsequently etching away a portion of the gallium arsenide to form independent islands of the semiconductor or device structure on the insulator, it is possible; e.g., to manufacture very high density integrated circuits in gallium arsenide, while still achieving optimal electrical isolation between adjacent elements. Similarly, such an island configuration further permits electrical isolation between microwave or other components formed on adjacent GaAs islands.
In addition, at certain temperature levels some of the electrically insulating substrates; e.g., BeO and sapphire, exhibit very good thermal conductivity. Thus, a GaAs device; e.g., an amplifier, on a BeO or sapphire substrate can be operated at a wider temperature range than a device fabricated in bulk GaAs. In contrast, bulk GaAs, as a substrate material, has a low thermal conductivity which results in heating of the device and hence an increase in the noise of the amplifier. Accordingly, by fabricating the device; e.g., an amplifier, of GaAs on BeO or sapphire, the heat would be conducted away by the substrate, with concomitant decrease of noise. It is also important, when devices made from III-V material are in a high temperature environment, for the insulator to act as a heat sink and dissipate the heat either externally added or generated by the device.
The present invention provides an improved technique for depositing a film of single crystal gallium arsenide or the like epitaxially on a single crystal substrate. The substrate may be either electrically insulating or non-insulating. The technique utilizes a reaction chamber having only one high temperature zone, and permits production of single crystal films of very high purity. Furthermore, the preferred inventive technique; e.g., in the formation of GaAs, utilizes either trimethylgallium or triethylgallium, which compounds overcome the prior art limitation, expressed by Lawley (op. cit.) when he stated that, "Gallium-arsenide films are harder to grow than elemental semiconductors because there are no suitable gallium compounds that are gaseous at room temperature and atmospheric pressure."
Also, in the process of formation of the semiconductor thin film by the described process, side reactions have been substantially decreased to provide significant reduction in the impurity level. Accordingly, a level of purity and quality may be attained whereby the single crystal films that are formed may be fabricated into "Gunn-effect" devices, for example.